In order to meet the increasing demands for wireless data traffic, wireless communication systems have been developed to support a higher data rate. For an increase in the data rate, the wireless communication systems have been evolved to improve the spectral efficiency based on the communication technologies such as Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple Input Multiple Output (MIMO).
Recently, the increase in demands for smart phones and tablet computers and the explosive growth in the number of applications requiring a large amount of traffic have accelerated the demands for data traffic. However, the tremendous demands for wireless data traffic may not be met only with the improvement of the spectral efficiency. Therefore, there is an increasing interest for a wireless communication system using a millimeter-wave band.
A system supporting wireless communication using the millimeter-wave band may suffer from an increase in the propagation loss such as path loss and return loss due to the frequency characteristics of the millimeter-wave band. Because of the increase in the propagation loss, the arrival distance of radio waves is reduced causing a reduction in coverage. Therefore, the wireless communication system using a millimeter-wave band has been considered as being implemented by using beamforming technology. When the beamforming technology is used, it is possible to increase the arrival distance of radio waves and the coverage by mitigating the path loss of radio waves. In other words, a millimeter-wave wireless mobile communication system needs to use the beamforming technology in order to mitigate high propagation loss in the millimeter-wave band. Furthermore, the beamforming technology needs to be applied to all cases in order to reduce mismatch between data and control signals.
The beamforming technology as suggested by the Institute of Electrical and Electronics Engineers (IEEE) 802.11ad: “Very High Throughput 60 GHz” includes two phases: Sector Level Sweep (SLS) and Beam Refinement Protocol (BRP). IEEE 802.1 lad is a Wireless Local Area Network (WLAN) standard that provides a very small service area with a radius of 10 to 20 meters in the 60-GHz millimeter-wave band. To overcome a wave propagation problem encountered with the millimeter-wave band, beamforming is used.
During the SLS phase, a Station (STA) that will perform beamforming transmits the same sector frame repeatedly in different directions and a peer STA receives sector frames through quasi-omni antennas and transmits feedback regarding a direction having the highest sensitivity. Therefore, the STA may perform beamforming by acquiring information about the direction having the highest sensitivity from the peer STA. During the BRP phase, Tx and Rx beam directions between the two STAs are fine-adjusted after the SLS phase in order to increase Tx and Rx beamforming gains. Generally, after the two STAs detect the best Tx beam during the SLS phase, they search for the best Rx beam matching the best Tx beam during the BRP phase.
Compared to the millimeter-wave wireless communication system, existing 2nd Generation (2G) to 4th Generation (4G) cellular communication systems are designed to transmit and receive control channels and data in a sub-1 GHz or 1 to 3 GHz frequency band in an isotropic or omni-directional fashion. However, some resources are optionally allocated to a user satisfying a specific channel condition by digital beamforming. Research has been conducted to achieve an additional performance gain by utilizing the multipath propagation characteristics of channels with Tx/Rx diversity based on multiple transmission and reception antennas, such as Multiple Input Multiple Output (MIMO), in the existing cellular systems.
Meanwhile, the multipath propagation of channels is mitigated due to the afore-described channel characteristics and use of transmit/receive beamforming in an extremely high frequency band like a millimeter-wave band. Therefore, a beamforming gain may be achieved but it is difficult to support Tx/Rx diversity. Accordingly, previous studies were limited to determination of a beamforming weight coefficient that optimizes a performance index such as Signal to Noise Ratio (SNR) by maximizing a beamforming gain during beamforming.
A wireless communication system using the afore-described beamforming technology may optimizes a performance index such as SNR by maximizing a beamforming gain. However, the wireless communication system using the beamforming technology hardly obtains a diversity gain since characteristics of the multipath propagation are decreased. In addition, the wireless communication system using the beamforming technology may become sensitive functionally for beamforming because of a mobile station's mobility or channel state, and beam information mismatch due to delay to actual allocation after beam measurement/selection. The wireless mobile communication system using the beamforming technology becomes sensitive to channel fading and obstacles due to strong directivity resulting from application of beamforming. Therefore, the wireless mobile communication system using the beamforming technology may use one or more beam patterns having different beamwidths and beam gains differently in consideration of channel states or characteristics of available resources.
Wireless Gigabit (WiGig), which does not support MIMO, is implemented based on beamforming through an analog array of a plurality of Radio Frequency (RF)/antenna elements, basically in one RF path or RF chain. For beamforming, a transmitter sweeps beams of a specific beam pattern in a plurality of directions and a receiver selects a beam having the largest signal strength and transmits feedback about the selected beam to the transmitter. This technique is generally applicable to an indoor environment having a Line of Sight (LoS) channel path in a short range of a few meters without mobility. In an outdoor wireless mobile communication environment characterized by mobility of tens of kilometers per hour, fast terminal switching, obstacle-incurred Non-LoS (NLoS) path characteristics, or a rapidly changing channel state caused by channel fading, beamforming that forms narrow beams having directivity, maximizing a beam gain in a specific direction may only increase sensitivity due to performance degradation attributed to the user environment. Therefore, a system may be designed in which one or more beam patterns having different beamwidths and beam gains are used differently in consideration of channel states or characteristics of available resources.
However, in a case of using one or more beam patterns having different beamwidths and beam gains, a gain difference between beams occurs in a specific direction due to trade-off between a beamwidth and a beam gain according to respective beam pattern. Therefore, there is a need to consider an operation of compensating for the beam gain difference according to a difference between beam patterns in actual link adaptation or uplink power control.